A method for changing an amorphous silicon film to a poly-crystalline silicon film includes the steps of irradiating an elongate pulse laser beam onto the silicon film while scanning in the direction normal to the major axis of the elongate pulse laser beam, to form a plurality of irradiated areas, irradiating flat-surface light onto the irradiated areas in the direction parallel to the major axis, and analyzing distribution of the reflected light from the irradiated areas to determine the threshold value of micro-crystallization. The threshold value is used to further determine an energy density of the elongate pulse laser beam for the phase change process.